The present invention relates generally to angular modulation optical transmission systems, and more specifically to a wavelength multiplexed optical transmitter.
Coherent light communications systems, in particular, the phase shift keying systems, are well suited for long distance communications purposes. Because of the coherent nature of the propagating waves, many different wavelengths can be multiplxed and transmitted on a single transmission medium over long distances.
To implement a high capacity, long distance optical transmission system, it is necessary to increase the output of laser as well as to increase the number of channels to be multiplexed. However, interchannel crosstalk occurs as a result of an incrdase in the number of such channels and in the intensity of light due to the nonlinear optical characteristic of the optical fiber.
More specifically, an intensity variation in the laser output of a given channel gives rise to phase variations, or noise in other channels due to the nonlinear characteristic of the transmission medium, causing a significant impairment of PSK signals. As described in Electronics Letter, Vol. 20, No. 24, pages 996-997, 1984, the following relation exists between a light intensity change in a given channel and a phase change in another channel: ##EQU1## where, .DELTA.P.sub.1 represents the light intensity variation of the source channel, and .DELTA..phi..sub.2 is the phase change of the affected channel, .omega..sub.1 represents the angular frequency of the signal, n.sub.2 is the nonlinear refractive index of the optical fiber (which is equal to 1.1.times.10.sup.-13 esu), Le represents the effective length of the optical fiber and is equal to (1-e.sup.-.alpha.L)/.alpha., (where .alpha.is the light loss coefficient of the optical fiber, L is the actual length of the optical fiber), n is the refractive index of the optical fiber, c is the velocity of light in vacuum, and Ae represents the effective cross-sectional area of the optical fiber (which is approximately the actual cross section of the core). Since the phase change .DELTA..phi. is proportional to the sum of the light intensity variations in other channels, the phase noise severely degrades the quality of a wavelength multiplexed-phase sensitive optical transmission system.
In order to avoid the crosstalk, one solution would be to prevent the variation of the output light intensity of the light source. However, even if use is made of a semiconductor laser which can generate an output having a small amount of light intensity variations and is driven by a constant current, the laser exhibits 1% output intensity variation (I.E.E.E. Journal of Quantum Electronics, QE-Vol. 19, pages 47-58, 1983, Yamamoto et al.). Further, an angular modulator, typically, ferroelectric material such as LiNbO.sub.3, is also not satisfactory since the angular modulation of light causes the light intensity to fluctuate in a range from several to 10% (The Journal of Optical Society of America, Vol. 2, No. 8 pages 1320-1326, 1985).